More restrictive exhaust emissions laws for modern engines (such as, for example, diesel engines) have driven changes in engine design, including the need for high-pressure electronic fuel injection systems. Engines built according to the new designs use higher combustion pressures, higher operating temperatures, and less lubrication than previous designs. Components of the new designs, including valve seat inserts (VSI), have experienced significantly higher wear rates. Exhaust valve seat inserts and valves, for example, must be able to withstand a high number of valve impact events and combustion events with minimal wear (such as abrasive, adhesive, and corrosive wear).
Another emerging trend in diesel engine development is the use of exhaust gas recirculation (EGR). In the EGR technique, exhaust gas is partially routed back into the intake air stream to reduce nitric oxide (NOx) content in exhaust emissions. The use of EGR in diesel engines can introduce harsher working conditions for valve seat inserts. Accordingly, there is a need for lower cost valve seat inserts having good hot hardness for use in diesel engines, such as diesel engines using EGR.
Additionally, exhaust gas contains compounds of nitrogen, sulfur, chlorine, and other elements that can potentially form acids. Acid can attack valve seat inserts and valves leading to premature engine failure. Accordingly, there is a need for improved corrosion resistance in alloys used in exhaust valve seat insert applications. Earlier attempts to achieve improved corrosion resistance were pursued through the use of martensitic stainless steels, but although these steels provide good corrosion resistance, conventional martensitic stainless steel do not have adequate wear resistance and hot hardness to meet the requirements for valve seat inserts in modern diesel engines.
There is a need in the art for improved iron-based alloys for valve seat inserts that exhibit adequate bulk hardness at elevated temperatures and compressive yield strength, as well as corrosion and wear resistance, which are suitable for use in exhaust valve seat insert applications.